1. Field of the Invention
The present invention relates to an image forming apparatus that corrects developing bias voltage.
2. Description of the Related Art
Conventionally, as a developing method for copy machines and printers using an electrophotographic technique, there has been employed a method in which a developing bias voltage formed by superimposing an AC voltage component, such as a sine wave voltage, a rectangular wave voltage, or a triangular wave voltage, on a DC voltage component is applied to a developing roller which is generally implemented as a developing sleeve containing a magnetic material (developing magnet). The DC voltage component mainly contributes to density of a developed image, and the AC voltage component mainly contributes to contrast of a developed image.
In this developing method, off-centering of a photosensitive drum, the developing roller (developing sleeve), and a spacer roller for holding a gap (SD gap) between the photosensitive drum and the developing roller sometimes causes periodic variation in the SD gap.
In this case, intensity of an electric field between the photosensitive drum and the developing roller periodically changes, which results in changes in density of a developed image.
As a solution to this problem, there has been disclosed a technique in which an AC component current of a developing bias is detected, and a DC component voltage of the same is sequentially changed according to the detected value of the AC component current, to thereby reduce density irregularity or variation caused by SD gap variation (see e.g. Japanese Patent Laid-Open Publication No. H09-54487).
Further, there has been disclosed a technique in which an image defect, such as density irregularity caused by SD gap variation, is reduced by performing FFT analysis of an AC current component of a detected developing bias to thereby extract a frequency component produced by off-centering of the photosensitive drum or the developing sleeve, calculating an opposite-phase component for offsetting the extracted frequency component, and superimposing an output of the opposite-phase component for offsetting the frequency component produced by off-centering, on the developing bias, at a timing shifted by a predetermined phase in synchronism with a drum rotation period during image formation (see e.g. Japanese Patent Laid-Open Publication No. 2008-287075).
However, in the image forming apparatus described in Japanese Patent Laid-Open Publication No. H09-54487, SD gap variation, as a cause of image density variation, is detected by the AC current component of the developing bias, and the DC voltage of the developing bias which changes image density is sequentially corrected, and hence image density variation can be corrected, but the AC current and the DC voltage of the developing bias have no direct correlation therebetween, and feedback control in this case does not form a feedback loop.
In other words, the feedback loop is not electrically closed, and hence if the amount of correction is increased, this increases a possibility of oscillation of the control, whereas if the amount of correction is reduced, this increases a possibility of an insufficient correction effect.
Further, although changes in the AC component current of the detected developing bias are sequentially corrected by correcting the DC voltage, the AC component current of the detected developing bias reflects not only variation caused by off-centering of the photosensitive drum or the developing sleeve but also variations caused by various factors. Therefore, this correction changes the DC voltage so as to correct even variations not required to be corrected, which can be a cause of unstable control.
Further, to perform FFT analysis of the AC current component of the detected developing bias to thereby extract the frequency component produced by off-centering of the photosensitive drum or the developing sleeve, as in the image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2008-287075, a complicated FFT analysis circuit is required, which can be a factor increasing the costs.